PROJECT SUMMARY Nephrotic syndrome (NS) is a poorly understood immune-mediated kidney disease. In children, 80% of all cases are steroid responsive and are referred to as steroid sensitive NS (SSNS), while the other 20% are steroid resistant (SRNS). SRNS is a major cause of end-stage kidney disease requiring dialysis and kidney transplantation; unfortunately, 60% of patients with SRNS will develop disease recurrence following transplant and ultimately kidney allograft failure. We and others have demonstrated that variants in Human Leukocyte Antigen (HLA) genes are associated with NS, signifying that defects in adaptive immunity, involving dysregulation of both T and B lymphocytes, may be central to NS pathogenesis. While the association between HLA and NS is strong, previous studies were carried out in small mono-ethnic cohorts using genome wide association chips with limited coverage of the HLA genes, and variants uncovered account for a small fraction of NS risk. Therefore, the precise HLA alleles/haplotypes associated with NS remain unknown. To enhance our understanding of NS, we and others have enrolled over 3,700 multi-ethnic patients with primary, secondary, and post-transplant NS from major multicenter kidney disease studies. We propose to perform state of the art next generation sequencing (NGS) of the coding and non-coding regions of major HLA class I and II genes to determine the relationship between variants in these genes and risk of NS, response to therapy, and disease recurrence following kidney transplantation. Our overarching hypothesis is that certain HLA alleles/haplotypes associated with NS can predict pattern of corticosteroid response in primary NS and the risk of disease recurrence following kidney transplantation. We will test our hypothesis and evaluate potential molecular mechanisms through the following aims: 1) Identify NS HLA risk alleles/haplotypes using high resolution HLA NGS in a cohort of multi-ethnic patients and determine the relationship between genotypes and therapy response, 2) Investigate the association between primary NS HLA risk alleles/haplotypes and secondary causes of immune-mediated NS (IgA and membranous nephropathy), 3) Determine common structural and functional motifs within NS HLA risk alleles/haplotypes and non-risk alleles by in-silico modeling and compare gene and protein expression of these alleles in B lymphocytes and kidneys of patients with NS, and 4) Determine the ability of known and novel NS HLA risk haplotypes to predict disease recurrence following kidney transplantation. Significance: The studies proposed in this application will use cutting-edge NGS of major HLA genes to identify the precise HLA alleles/haplotypes that are associated with NS, therapy response, and disease recurrence following kidney transplantation. Understanding the intrinsic role of HLA variants and adaptive immunity dysfunction in NS will lead to identification of novel pathways that are important in disease pathogenesis and therapy not just for NS, but also for other common glomerular diseases.